1. Field of the Invention
The present invention relates to a color image processing apparatus that can be suitably adapted to a full color reproducing apparatus and like apparatus, in order to improve color reproduceability while preventing the quality of the recorded image from being deteriorated by the density shading of markers entered in a document.
2. Description of the Prior Art
There has been proposed a color image processing apparatus in which color images such as character image, photograph image and like images are optically read and divided into red R, green G and blue B, and are converted into recording colors such as yellow Y, magenta M,cyan C and black K, which are then recorded onto a recording paper using an output apparatus such as an electrophotographic color image reproducing machine.
There has further been proposed a color image processing apparatus of this type but which has a marker color conversion function to convert a portion surrounded by a marker into a color same as the marker among black characters on a black-and-white document.
With such color image processing apparatuses, the image data of input document are usually converted into signals of R, G and B. However, the recording colors of the printer unit are C, M, Y and K which are complementary colors thereof.
In this case, the spectral sensitivity characteristics of a scanner and the spectral reflection factor of a toner differ from each other as shown in FIGS. 29(A) and 29(B). Therefore, the density levels of R, G and B found based upon the scanner level can be converted into density levels of C, M and Y toners by the linear masking method.
Here, the linear masking is expressed by the following equation, ##EQU1## Dr, Dg, Db--R, G, B brightness levels of the scanner that are converted into density levels,
Dc, Dm, Dy--C, M, Y toner adhesion amounts that are converted into density levels, PA1 aij (i, j=1, 2, 3)--matrix coefficient.
As will be obvious from the L*a*b* isochromatic coordinate system of FIG. 30, the R, G and B (original colors) are in nearly perfect agreement with C, M, and Y after recorded (copy). In regard to other colors, however, conversion error becomes conspicuous since the linear masking method is an approximation.
Therefore, if the masking coefficient is calculated using three colors R, G and B, the color reproduceability is deteriorated in the printer system.
In a marker color conversion circuit used for such a color image processing apparatus, furthermore, the region of a marker MC and its color entered in the document, as shown in FIG. 27, are read for each scanning line to determine the region and the marker color for each scanning line. In FIG. 27, dots "." represent sampling points for determining the marker color.
When the marker color conversion function is utilized, on the other hand, it is often attempted to change the recording density of image included in a region surrounded by the marker MC depending upon the density of the marker. In such a case, however, a change in the density of the marker MC turns out to be shading in the recording density to deteriorate the quality of recording.
Further, when a certain region is surrounded by the marker MC as shown in FIG. 28, the upper half of the marker being, for example, red and the lower half thereof being blue, the upper half of the image in the region surrounded by the marker is recorded in red which is the same color as the marker color and the lower half is recorded in blue.
In this case, it is better to record the image in the same color as the initial color of the marker MC for better visibility.